The present invention relates to a button insert for rock drill bits comprising a base portion intended to be secured in the body of the drill bit and a unit integral with the base portion constituting a cutting tip.
The button insert is primarily intended to be used in connection with rotating drill bits, i.e. roller bits and bits for raise boring and tunnelling. However, the button insert could also with preference be used with drill bits for percussion drilling.
At present a frequent insert design in rotating bits has a cylindrical base portion and a substantially conical cutting tip, the sharp edges of the truncated cone being trued thereby providing a relatively blunt tip. By another frequent insert design, said inserts have a substantially semi-spherical cutting tip. These two button insert designs are for instance illustrated in U.S. Pat. No. 4,375,242. The first-mentioned button insert design is mostly used when drilling in soft and medium hard formations while the last-mentioned button insert design mostly is used when drilling in hard formations.
There are many modifications of these two basic designs. A common modification of the truncated, conical cutting tip is that opposite sides of the cutting tip are planed thereby providing a chisel-shaped cutting tip. A button insert design of this type is for instance illustrated in U.S. Pat. No. 4,406,337.
In U.S. Pat. No. 4,254,840 a button insert design is disclosed that makes an attempt to unite the characteristics of semi-sperical and chisel-shaped cutting tips.
A special modification of the chisel-shaped cutting tip is disclosed in U.S. Pat. No. 4,108,260 having the face directed forward in the direction of revolution concave and the opposite face convex. By this button insert design a high bending resistance is pursued and this is achieved by the extra support that is given because of the convex face and due to the concave face being designed simultaneously to raise the cuttings.
Still another button insert design for soft and medium hard and also hard formations is disclosed in U.S. Pat. No. 3,388,757, the button insert being provided with a number of planar or slightly concave bevellings that are separated by crests, said crests being intended to act as cutting edges during drilling.
In German application No. 3,317,441 a cutting insert is disclosed intended to be used in tools for breaking solid materials, e.g. asphalt. This cutting insert comprises a conical cutting tip and a shoulder intended to abut against the tool body. Between the cutting tip and the shoulder there is an intermediate portion comprising a concave portion. The intermediate portion is intended to maintain the required cutting force at a low level even when the cutting tip is worn.
The present invention should be regarded as a modification of a button insert having a truncated concical cutting tip.
The aim of the invention is to provide a button insert that is subjected to a more evenly spread stress caused by a lateral load compared to the conditions of button inserts having conventionally designed conical cutting tips.
Another aim of the invention is to, when comparing the conditions in button inserts having conventionally designed concical cutting tips, reduce the required force to penetrate the button insert to a given depth at both straight as well as inclined penetration of the button insert into the formation.
Still another aim of the invention is to, when comparing the condition of a button insert having conventionally designed conical cutting tips, reduce the required force to displace the button insert perpendicular to its longitudinal axis after the penetration in the formation.
Further aims are to achieve a button insert having generally seen concical cutting tip shape that clears from the formation as fast as possible after the previous penetration, is worn blunt slowly in order to maintain a high penetration rate for a long time, allows a large, accessible free space for cuttings around the button inserts, and in rotational drilling, between the cone and the formation for providing an effective flushing, a smaller amount of hard material is required thereby making the button insert inexpensive and also that the breaking stress caused by bending relatively seen is increasing due to the smaller amount of hard material.
The invention is described more in detail in the following with reference to the accompanying drawings disclosing an embodiment by way of example. Said embodiment is only intended to illustrate, the invention that can be modified within the scope of the claims.